Advances in Thermionic Energy Conversion through Single-Crystal n-Type Diamond

At a Glance

Metadata	Details
Publication Date	2017-12-06
Journal	Frontiers in Mechanical Engineering
Authors	Franz A. Koeck, R. J. Nemanich
Institutions	Arizona State University
Citations	23
Analysis	Full AI Review Included

Technical Analysis: Advances in Thermionic Energy Conversion using Single-Crystal n-Type Diamond

This document analyzes the research paper “Advances in Thermionic Energy Conversion through Single-Crystal n-Type Diamond” (Koeck and Nemanich, 2017) to provide technical documentation and specific material solutions offered by 6CCVD.

Executive Summary

The research validates Single-Crystal Diamond (SCD) as a superior material for high-efficiency Thermionic Energy Converters (TEC) by demonstrating unprecedented control over the electron emission barrier (work function, $\phi$).

Ultra-Low Work Function Achieved: MPCVD-grown, Phosphorus (P)-doped SCD electrodes achieved a thermionic work function ($\phi$) as low as 0.67 eV when terminated with hydrogen, one of the lowest values ever reported for a thermionic material.
Efficiency Potential: This ultra-low work function approaches the theoretical ideal collector work function (0.5 eV) required for TEC efficiencies exceeding 50%, significantly surpassing conventional thermal power plants.
Material Engineering: Work function control (ranging from 2.88 eV down to 0.67 eV) was achieved by precisely tuning the n-type doping concentration (Phosphorus and Nitrogen) and utilizing the Negative Electron Affinity (NEA) induced by hydrogen passivation.
Mechanism Confirmation: The study established a direct relationship between doping concentration, upward band bending, and surface state density ($N_{ss}$), confirming that precise doping is critical for minimizing the emission barrier.
Device Feasibility: A prototype TEC cell using N-doped SCD (emitter) and P-doped SCD (collector) demonstrated thermionic current flow, confirming diamond’s viability for high-temperature, solid-state TEC applications.
Methodology: SCD films were grown homoepitaxially via Plasma-Enhanced Chemical Vapor Deposition (PECVD), followed by hydrogen passivation and high-temperature annealing (up to 1,050 K).

Technical Specifications

The following hard data points were extracted from the experimental results concerning SCD material properties and thermionic performance.

Parameter	Value	Unit	Context
Band Gap ($E_g$)	5.47	eV	Intrinsic Diamond
Ideal Collector $\phi$	0.5	eV	Required for >50% TEC efficiency
Lowest Work Function ($\phi$)	0.67	eV	P-doped SCD (100), H-passivated
N-Doped Work Function ($\phi$)	2.88	eV	N-doped SCD (100), H-passivated
P-Doping Concentration ($N_d$)	2 x 10¹⁷	cm^-3	Low-doped P-SCD (10 nm film)
N-Doping Concentration ($N_d$)	3.3 x 10¹⁹	cm^-3	High-doped N-SCD (Emitter)
Richardson Constant ($A_R$)	68	A cm^-2 K^-2	N-doped SCD (100)
Richardson Constant ($A_R$)	2.3 x 10^-7	A cm^-2 K^-2	Low P-doped SCD (100)
P-Doped Film Thickness	10	nm	Low P-doped layer
High P-Doped Film Thickness	~275	nm	Higher P-doped layer
TEC Operating Temperature	780 - 950	K	Thermionic emission testing range
Emitter Annealing Temperature	1,050	°C	Hydrogen desorption study
Surface State Density ($N_{ss}$)	3 x 10¹¹	cm^-2	Low-doped homoepitaxial films
Electron Mobility	4,500	cm² V^-1 s^-1	Intrinsic Diamond Property

Key Methodologies

The n-type SCD electrodes were prepared using Plasma-Enhanced Chemical Vapor Deposition (PECVD) and specific surface treatments to induce Negative Electron Affinity (NEA).

Substrate Preparation: Commercially available High-Pressure, High-Temperature (HPHT) Type Ib (Nitrogen-doped) SCD plates, primarily (100) and (111) orientations.
Wet-Chemical Cleaning: Multi-step cleaning including boiling in H₂SO₄/H₂O₂/H₂O (3:1:1) at 220°C, followed by HF and NH₄OH/H₂O₂/H₂O (1:1:5) at 75°C.
PECVD Growth (Phosphorus Doping):
- Reactor: Dedicated PECVD system with base pressure in the low 10^-8 Torr regime.
- Gas Mixture: Research grade H₂, CH₄ (2 sccm), and Trimethylphosphine (TMB) in H₂ (10 sccm) as the P-source.
- Parameters: Microwave Power (2,500 W), Pressure (85 Torr), Substrate Temperature (950°C).
Hydrogen Passivation (NEA Induction): Pure H₂ plasma exposure (400 sccm H₂ flow, 65 Torr, 1,300 W, 850°C) for 5-15 minutes.
Thermionic Characterization: Electron emission current measured as a function of temperature (780 K to 940 K) in a low 10^-10 Torr vacuum, fitted to the Richardson-Dushman relation.
Surface State Control: High-temperature annealing (up to 1,050°C) was used to desorb hydrogen, resulting in a transition from NEA (low $\phi$) to positive electron affinity (higher $\phi$).

6CCVD Solutions & Capabilities

6CCVD is uniquely positioned to supply the advanced MPCVD diamond materials required to replicate, optimize, and scale the high-efficiency thermionic energy conversion technology demonstrated in this research.

Research Requirement	6CCVD Solution & Capability	Value Proposition
Material: Single-Crystal n-Type Diamond (P/N doped)	Custom Doped SCD: We specialize in high-purity Single-Crystal Diamond (SCD) substrates and homoepitaxial growth. We offer custom doping recipes for precise Phosphorus (P) and Nitrogen (N) incorporation, essential for engineering the donor levels (0.6 eV for P, 1.7 eV for N).	Enables precise control over the work function ($\phi$) and band bending ($W_d$), critical for achieving ultra-low emission barriers (0.67 eV).
Thickness Control: Films from 10 nm to 275 nm	Precision Thickness Control: SCD layers are available from 0.1 µm up to 500 µm. This capability allows researchers to precisely define the depletion width ($L_d$) and optimize the doping profile gradient, which directly impacts the surface state density ($N_{ss}$).	Supports advanced device modeling and optimization of the emitter/collector junction characteristics.
Surface Quality: Ra < 10 nm required for NEA stability	Ultra-Smooth Polishing: Our standard SCD polishing achieves surface roughness of Ra < 1 nm, significantly exceeding the requirements of this study. PCD plates are available with Ra < 5 nm up to inch-size.	Ensures optimal surface termination (e.g., hydrogen passivation) stability and minimizes scattering losses in the vacuum gap.
Electrode Contacts: Need for high-quality electrical contacts	Custom Metalization Services: We offer in-house deposition of standard and custom metal stacks (Au, Pt, Pd, Ti, W, Cu). This is crucial for mitigating the Schottky barrier effects observed at the collector contact, potentially through selective growth of highly doped diamond regions.	Streamlines device fabrication and ensures robust, low-resistance electrical contacts for high-temperature operation (up to 950 K).
Scaling & Dimensions: Custom (100) and (111) orientations	Large Area & Custom Dimensions: We supply SCD and Polycrystalline Diamond (PCD) plates up to 125 mm in diameter, with precise orientation control. The (111) orientation, noted for better P incorporation, is readily available.	Supports scaling from R&D prototypes to commercial-scale TEC modules.
Engineering Support	In-House PhD Team Consultation: 6CCVD provides expert engineering support for material selection, surface termination strategies (e.g., hydrogen passivation), and doping optimization for similar Thermionic Energy Conversion projects.	Accelerates R&D timelines and ensures the selection of the optimal diamond grade (SCD or PCD) for specific application requirements.

For custom specifications or material consultation, visit 6ccvd.com or contact our engineering team directly.

View Original Abstract

abstract: Thermionic energy conversion, a process that allows direct transformation of thermal to electrical energy, presents a means of efficient electrical power generation as the hot and cold side of the corresponding heat engine are separated by a vacuum gap. Conversion efficiencies approaching those of the Carnot cycle are possible if material parameters of the active elements at the converter, i.e., electron emitter or cathode and collector or anode, are optimized for operation in the desired temperature range. These parameters can be defined through the law of RichardsonâDushman that quantifies the ability of a material to release an electron current at a certain temperature as a function of the emission barrier or work function and the emission or Richardson constant. Engineering materials to defined parameter values presents the key challenge in constructing practical thermionic converters. The elevated temperature regime of operation presents a constraint that eliminates most semiconductors and identifies diamond, a wide band-gap semiconductor, as a suitable thermionic material through its unique material properties. For its surface, a configuration can be established, the negative electron affinity, that shifts the vacuum level below the conduction band minimum eliminating the surface barrier for electron emission. In addition, its ability to accept impurities as donor states allows materials engineering to control the work function and the emission constant. Single-crystal diamond electrodes with nitrogen levels at 1.7 eV and phosphorus levels at 0.6 eV were prepared by plasma-enhanced chemical vapor deposition where the work function was controlled from 2.88 to 0.67 eV, one of the lowest thermionic work functions reported. This work function range was achieved through control of the doping concentration where a relation to the amount of band bending emerged. Upward band bending that contributed to the work function was attributed to surface states where lower doped homoepitaxial films exhibited a surface state density of â¼3 Ã 10[superscript 11] cm[superscript â2]. With these optimized doped diamond electrodes, highly efficient thermionic converters are feasible with a Schottky barrier at the diamond collector contact mitigated through operation at elevated temperatures.

Tech Support

Original Source

DOI: https://doi.org/10.3389/fmech.2017.00019

References

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